1. Field of the Invention
The invention is generally directed to high purity iron-zinc alloys and a method of preparing the alloys. More particularly, the invention is directed to the preparation of iron-zinc alloys of calibration standard quality.
2. Description of the Prior Art
The demand to improve the corrosion resistance of steel sheet, particularly for use within the automotive industry, has led to a dramatic increase in the use of coated steels in place of ordinary cold-rolled sheet steel. Consequently, industrial interest in the processing of zinc alloy coatings has risen over the past decade. Hot-dip galvanized and galvannealed sheet steel are two products in use today. These processes involve the use of zinc and zinc-alloy coatings to protect the steel through the sacrificial or galvanic mechanism. They are an economic way to apply the zinc, and today's continuous coating lines are capable of producing materials having well controlled coating thickness and uniformity.
Galvanneal steel differs from galvanized steel in that it results from the post annealing of the zinc-coated steel sheet, thereby allowing the interdiffusion of the iron and zinc to form an iron-zinc intermetallic coating. The iron content in the coating depends primarily on the anneal temperature and time. Within this alloy coating, the four different iron-zinc phases, Zeta, Delta, Gamma-1, and Gamma, may be present. In order to manufacture the most advantageous coating, it is important to identify which phases form during the galvannealing process, to understand the properties of each phase, and to know how to control the formation of any particular phase or phases in order to obtain optimal material performance. Positive identification of each phase and the fraction present in a galvanneal coating is very difficult. The primary cause of this difficulty is the lack of high quality data on the crystal structure and the related microstructure of the separate iron-zinc phases.
Although several publications discuss the preparation of some iron-zinc intermetallics (see, Baskin et al., Z. Metallkde, 68, 359 (1977), and Gellings et al., Z. Metallkde, 70, 312 (1979)), to date, none report on the preparation of the alloys having high purity and homogeneity. It would be advantageous to produce very pure and homogenous iron-zinc alloys which are useful as instrument calibration standards, such that galvanneal coatings can be more effectively evaluated.